LATE LADINIAN (MIDDLE TRIASSIC) MAJOR ECOLOGICAL EXPANSION INTO THE OPEN OCEAN-EVIDENCE FROM THE TRIASSIC MARINE REPTILE FAUNAS FROM SOUTH CHINA

After the end-Permian mass extinction, marine reptiles first appeared in the middle Spathian (Olenekian, Early Triassic), soon after the end Smithian lethally hot greenhouse and quickly became the top predators of the Mesozoic marine ecosystem. The Chaohu Fauna, yielding the oldest basal ichthyopterygian Chaohusaurus and eosauropterygian Majiashanosaurus, is a marker of this occurrence. The Middle Triassic is a period of rapid biotic radiation for the Mesozoic marine reptiles, indicated by the east Tethyan Panxian and Luoping faunas (Pelsonian, Anisian), the west Tethyan Monte San Giorgio and Germanic faunas, and the west North American Nevada fauna, yielding mainly coastal and small-medium body sized species including Mixosaurus, Phalarodon, Placodus, Nothosaurus, Lariosaurus, and protorosaurs Tanystropheus and Macrocnemus, and saurosphargids. After the radiation, oceanic cruisers, such as euichthyosaur ichthyopterygian and pistosauroid sauropterygian evolved and became the “ruler” of marine ecosystem in the Jurassic and Cretaceous. This major change of marine reptiles probably co-occurred along with the end Ladinian global regression.

A 5.9m thick fossiliferous level of the Late Ladinian (Middle Triassic) Xingyi Fauna, yields 15 or more taxa of marine reptiles and can be subdivided into two assemblages. The lower one contains mostly coastal taxa including the pachypleurosaur Keichousaurus, the nothosaurians Nothosaurus and Lariosaurus, the protorosaurs Macrocnemus, Fuyuansaurus and Tanystropheus, and the archosaur Diandongosuchus, as well as large semidurophagous halecomorph fishes, representing a near-shore ecosystem with strong western Tethyan affinities. The upper assemblage yields the large shastasaur Guizhouichthyosaurus and the medium-sized euichthyosaur Qianichthyosaurus with a distinct tailbend, both phylogenetically close to their respective North American sister taxa that are younger. The pistosaur Yunguisaurus, which is phylogenetically close to plesiosaurs, and the oldest “flying fish” Thoracopterus also comes from this bed, together with large paleonisciform fishes showing light scale covering. This fauna suggests fast ecological shift, incurring a major transition from coastal to open ocean environment.